A sloper is a basic fitting garment (also called a body block) - it is made to fit a particular body
(or doll) from a set of measurements. It has little or no wearing ease and
has the proper placement and size of darts - and usually does not have seam allowances
(seam allowances are added after design elements are completed).
It is created by either the flat pattern method (using a set of measurements and calculations)
or by draping techniques. A sloper can be a bodice, skirt or pants. It becomes a basic pattern
used to create different designs (via dart manipulation or slash & spread) while retaining a proper fit.
If you have a sloper for the doll you are creating a pattern for, the sloper can be used to help
determine the correct fit of the new pattern as well as checking for proper dart placement.
Below is an example of a human bodice sloper:

DARTS are used to create shaping around parts of the body such as the breasts, derriere and the stomach. The more round the body part is, the wider (larger) the dart wedge is.

What does "manipulating the darts" mean? It basically means moving the darts around the
sloper bodice to create different style-looks (the darts can be moved to any other
part of the bodice - for more information reference any pattern drafting book).
The picture below shows how one vertical waist dart can be moved around a
bodice to create different looks without changing the fit
(taken from Patternmaking for Fashion Design, Fourth Edition, Helen Joseph-Armstrong, 2006):

One very common dart manipulation is slashing the sloper bodice and closingthe darts to create a princess seam style line
(theoretically you would do this using an actual set of measurements):

Step 1: Determine where center front to the bust point is on the bodice front pattern piece (also
called the apex - blue dot). Use the bust-point-to-bust-point measurement
and divide it in half to determine the center to bust point measurement- red lineA
(e.g., Elfdoll small-busted Rainy doll measures 2" bust-point-to-bust-point - half of that would be 1").
A vertical mark would be placed on the bodice pattern piece at 1" from the center front.Step 2: Measure and place a mark on the pattern at the shoulder-to-bust-point - using
the measurement from the measurement sheet - shown with green line B.
The point where the red and green line intersect is the bust point - blue dot. Step 3: On bodice back, draw a curved line from the tip of the shoulder dart to top tip of waistline dart (shown with orange line - C).

These markings determine the first cutting lines:

The wedge pieces of each dart are cut away and cuts are made along the green and
orange cuting lines to create front/side front and back/side back pieces.
Another cut is made from the bust point to the tip of the side dart
(see black dashed arrow in picture above).

After the cuts are made, separate the pieces. The cut that is made from the bust point to the tip
of the dart allows the side dart to be closed and taped together creating the curve of the side front.

The curve on the side front piece is smoothed out - there area where the wedge appears will be eased to the front piece when sewing the seams together. Seam allowances are added where shown.

NOTESon dart placement: Knowing the how and why of dart placement is very important when creating a pattern for
a mature, female figure doll. Darts are not randomly placed on a pattern piece - rather,
there are specific rules as to where they are placed and how long they are.
You may need to move the darts if using a pattern that was designed to fit a doll that
is different than yours. Additionally, if you are enlarging the pattern instruction pictures
(of a people pattern) to create a pattern, there is no clear indication of where to place the darts.
So knowing the specifics of dart placement will help you in all of your doll pattern creating and sewing.

DART PLACEMENT for Bodice Front and Back Pieces

BODICE FRONTOn bodice front pattern piece you would locate and mark the the bust point as outlined
above (in description of creating a princess line bodice). The dashed red line thru the
front bodice darts on the above picture indicates where the center of the side and
vertical darts are - and note they point to the bust point. However, the bust darts
should never, ever "meet" (come together) at the bust point (nipple). They both
always stop at a determined distance away from the bust point - yet always point
to the bust point (see circle drawn around the bust point). On humans the stopping
point for the darts (circle diameter) is 1-1/4 to 1/-1/2" from the bust point. The
distance is determined by cup size - the smaller the breasts the closer the darts
come to the bust point; the larger the breasts, the further away the darts are from
the bust point (think of fitting around a smaller or larger round ball - the closer the
darts are to the bust point, the more "pointed" a shape the darts create).

BODICE BACKOn bodice back the vertical waist dart aligns with the shoulder blades. The top of the dart should not extend above the top of the vertical bust dart (see green dashed line).

The following pictures show how changing the dart location will change the look (and the fit) of the bodice front (note these were all cut from the same sloper pattern and photographed on the same doll - the difference if the look of the fit is created by the changes in the dart placement):

Bust darts too close to bust point
(and too close together)

Both side and vertical bust darts do not
point towards or line up with bust point (black circle).

Both bust darts line up with bust point (black circle) and are a proper distance from the bust point.

Note on sewing darts: Darts should be stitched from the widest part of the wedge to the tip. To achieve a crisp point, change to a smaller stitch as you approach the tip and sew off the end of the dart (do not backstitch) sewing a few stitches off the end to lock the stitches. Cut thread leaving about a 1/4" tail.

PATTERN ORIGINSUnderstanding as much as you can about how commercial patterns are created will help you to
figure out what alterations might be needed to create a pattern that will fit the doll you are designing for.

All of the major commercial patterns (McCalls, Butterick, Simplicity, Vogue & Burda) are created using a standardized sloper. In other words, the pattern company has a uniform set of measurements for each size pattern and it is used to create all of the different designs. Knowing what these measurements are - and some other features of the pattern - is very helpful in working with "where the pattern came from" in making alteration and ease decisions. The main body measurements that the companies use is clearly listed on each pattern envelope - so there is no guessing. Additionally, reference points such as the bust point, hipline, waistline, etc. are always in the same place. Many times these are marked on the pattern - there will be a notation of the actual measurement and a circle with an X in the middle of it marking the place where the bustline, hipline and other pertinent fitting areas are. More importantly, the slopers are drafted to fit a B-cup bust on a person. I have consistently found that I have to make the full bust alteration when creating patterns for dolls (from people patterns) - even for those that are classified as having a "regular" bust size (e.g., "small" bust Elfdoll Rainy dolls). It isn't always the "fullness" of the bust - but can also occur because of the erect posture, uplifted shape of the breast and the fact that the resin does not squish or move when placed in a garment.

There will also be times that you won't know what the source of the pattern is - this is particularly so in the case of period costume books. Many times the patterns have been copied from actual period garments and once enlarged (most are scaled down in the book so enlargement is required to fit a human) they will not fit a modern body - human or doll. For example, if the garment is meant to be worn over a corset, the person in that period has most likely worn a corset for many years and their actual body shape has changed from the wearing (and the bustline is usually flattened as well). Therefore, a pattern from that time period will not ever fit correctly (without alteration) on a modern body (even if corseted). And with dolls, there is no ability to change the waist or bust shape & size with a corset, bra or other kind of body shaping garment. In other cases the book of patterns you are using to create a doll pattern may not give you any type of scale measurement on which the patterns are based. Therefore you will need to know how to measure the pattern pieces themselves (and figure out how much ease is possibly incorporated into the pattern) to determine how much enlargement or shrinkage is needed to create a pattern to fit your doll.